CMRR for differential interconnects

CMRR (common-mode rejection ratio) for a measured pair is how much stronger the differential transmission is than the common-mode transmission:

CMRR_linear = |SDD21| / |SCC21|
CMRR_dB     = 20 * log10(|SDD21| / |SCC21|)
            = SDD21_dB - SCC21_dB

SDD21_dB and SCC21_dB are 20*log10(|S|) (usually negative for a cable). Subtracting them yields a positive CMRR in dB when |SCC21| is smaller than |SDD21|.

Example: SDD21 = −2 dB, SCC21 = −42 dB → CMRR = 40 dB.

Do not take absolute values of the dB traces and subtract those; that sign-flips the result.

What 40 dB means

40 dB means |SCC21| is 100× smaller in voltage-wave amplitude than |SDD21| (10⁴ in power). For many digital / high-speed cables that is a solid figure. Whether it is “good” depends on the spec in force:

Rough band (cable SDD21 vs SCC21)	Reading
< 20 dB	Poor balance or a mode-conversion problem — investigate
20–30 dB	Often acceptable for short, non-critical runs; check the standard
~40 dB	Comfortable for a wide range of cable/connector work
> 50 dB	Excellent; also check that SCC21 is not in the instrument noise floor

A requirement of “40 dB min from 100 MHz to 6 GHz” is a spec, not a universal truth. Always compare against the project’s limit line. CMRR that is huge because SCC21 is the VNA’s noise floor is not a balance measurement — look at the raw |SCC21|.

CMRR is not SDC/SCD

Mode-conversion traces (SDC21, SCD21) describe conversion between modes. CMRR as defined above is a ratio of like-mode transmissions. A part can have decent CMRR and still fail a mode-conversion mask. Report the quantity the question asked for.

How to compute it in practice

1. Confirm port mapping.
2. Form SDD21 and SCC21 in complex linear (0.5 formulas or an S4P).
3. Convert each to dB, subtract.
4. Publish CMRR as its own trace if the team plots it next to SDD21.

A single 2-port file cannot produce CMRR.